The invention is based on a fuel injection device, in particular a unit fuel injector for internal combustion engines as defined hereinafter. In a fuel injection device of this kind, known from an earlier German patent application P 41 42 998.2-13, a pump piston axially guided in a cylinder bore of a pump housing is driven to reciprocate by a cam drive. With its face end remote from the cam drive the pump piston defines a pump work chamber in the cylinder bore into which a fuel supply line discharges and which via a pressure conduit communicates with an injection valve protruding into the combustion chamber of the engine to be supplied. Thus not only the onset of the high-pressure delivery of the fuel found in the pump work chamber and therefore the onset of injection, but also the quantity of fuel to be injected is regulated via the diversion process by means of a magnet valve, disposed in the fuel line, which is controlled as a function of the operating parameters of the engine to be supplied.
To this end, the magnet valve has an electrically triggered valve member, which rests against a valve seat in the valve body with a conical valve sealing face disposed on its circumference. In the absence of current, the magnet valve is open; not until there is a supply of current does it bring the valve member into contact with the valve seat, counter to the force of a valve spring, and cause it to close. For the sake of the most minimum possible design of the magnet valve adjusting magnet and of the valve spring, the valve member has an annular cross-sectional constriction at its circumference, at the level of the entry of the high-pressure line from the pump work chamber, and this constriction is furthermore located in an annular chamber in the valve body when the magnet valve is closed, so that the fuel can flow evenly around the valve member, and so that high fuel pressure will act equally on the valve member in both the opening and closing directions of the valve member. As a result, the adjusting forces can be kept correspondingly small.
To cool the magnet valve in the known fuel injection device, a flow of force at low pressure, which is taken from the low-pressure chamber disposed on the underside of the magnet valve via a respective connecting conduit, each of which has a throttle, flows through part of the magnet chamber and immediately thereafter returns to a chamber having a low level of pressure.
The magnet valve of the known fuel injection device has the disadvantage, however, that on high-pressure entry, the valve member is put under a great deal of hydraulic stress at the annular cross-sectional constriction; in both the opening and closing direction of the valve member, the high axial forces acting upon the involved transition surfaces of the cross-sectional constriction can exert a concentration of stress upon the remaining cross section at the narrowest part of the valve member there, which can lead to a fatigue fracture.
If such a pressure occurs, the high axial forces drive the parts of the valve member apart at the point of fracture; the high pump work pressure now acts upon the entire valve member cross section and consequently holds the valve member with its sealing face pressed against the valve seat. The opening force of the valve spring is now no longer sufficient for the valve member to open independently, so that it remains closed throughout the entire stroke of the pump piston, and as a result, the fuel injection device injects the maximum possible supply quantity into the combustion chamber of the engine. This uncontrolled, uncheckable high fuel injection quantity can then lead to an increase in the engine speed above the permissible range, which can eventually destroy the engine.